Wireless Local Area Networks (WLANs) have become extremely popular allowing users freedom to connect to a network without the inconvenience of a network cable. The market for wireless communications has grown significantly since the introduction of WLANs, and with the relative ease of integrating wireless access with existing network resources including printers, servers and Internet connections.
The most obvious differences between WLANs and wired networks such as Ethernet are those imposed by the transmission medium. Whereas Ethernet sends electrical signals through wires, WLANs send radio frequency energy through the air. Wireless devices are equipped with a special network interface card (NIC), having an antenna and a radio transceiver and associated circuitry for converting between the analog radio signals and the digitally encoded data.
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 WLAN standard is by far the most popular and widely deployed wireless LAN (WLAN) technology. The original IEEE 802.11 standard was published in June 1997 and specified a physical layer (L1/PHY) and medium access control layer (L2/MAC) for interoperable WLAN operation. The original standard was intended to operate in the unlicensed Industrial, Scientific, and Medical (ISM) band at 2.4 GHz and supports a bit rate of 1 Mbps and an optional higher rate of 2 Mbps. In September 1999 the IEEE approved a HR or “high rate” extension to the standard, known as the IEEE 802.11b, which supports data rates up to 11 Mbps. More recently, another extension known as the IEEE 802.11a has emerged that supports data rates up to 54 Mbps but in a higher frequency ISM band at 5 GHz. Another extension that has been ratified is the IEEE 802.11g that supports data rates of 54 Mbps but in the original 2.4 GHz ISM band.
The logical link control layer (L2/LLC) is defined by the IEEE 802.3 standard and as a result IEEE 802.11b WLANs are fully compatible with wired Ethernet LANs. In fact the IEEE 802.11b is intended to provide wireless access to a wired Ethernet backbone through access points (APs) and station adapters (STAs).
The Basic Service Set (BSS) is the fundamental building block of the IEEE 802.11 architecture. A BSS is defined as a group of STAs that are under the direct control of a single coordination function. The geographical area covered by the BSS is known as the Basic Service Area (BSA). A BSA is analogous to a “cell” in a cellular communications network.
The IEEE 802.11 standard supports two modes of operation. The first of these is the infrastructure mode that uses a cellular architecture and employs an AP to coordinate the management of the cell and to act as a gateway to a network infrastructure. The other mode of operation is the peer-to-peer or “ad hoc” mode where STAs can communicate directly with one another on a peer-to-peer basis without the use of an AP. This gives rise to what is known as an independent basic service set (IBSS).
The 802.11 standard defines two fundamentally different Medium Access Control (MAC) schemes to transport asynchronous and time-bounded services. The first of these is the distributed coordination function (DCF) used to support asynchronous data transfer on a best effort basis where all STAs must contend with each other to access the medium in order to transmit their data. The DCF allows multiple STAs access the medium without the need for central control and employs a technique known as carrier sense multiple access with collision avoidance (CSMA/CA). The DCF is implemented in the AP and all STAs in both infrastructure and IBSS network configurations. The CSMA/CA mechanism is designed to reduce the collision probability between multiple STAs attempting to access the medium at the same time. To improve reliability all directed (i.e. unicast) traffic uses immediate positive acknowledgment where a re-transmission of the frame is scheduled by the sender if no acknowledgment is received. The present invention is primarily directed at this mechanism. In this context, it will be appreciated that both APs and STAs may be regarded as individual stations since they both contend for access.
The other MAC scheme is the Point Coordination Function (PCF) and is based upon polling. It is designed for the transmission of delay sensitive traffic. The PCF is an optional capability that has not generally been implemented by any of the major manufacturers.
The functionality of the 802.11 standard is reflected in the frame headers. RF technology and STA mobility impose complex requirements on 802.11b WLAN networks. This added complexity is reflected in the PHY and MAC headers.
In order for a network manager to ensure the efficient operation of the network, it is necessary for them to be able to assess the operation of the network. To date there has only been limited capability or a limited number of devices available to assist a network manager in this regard, including WLAN probes.
A variety of WLAN probes are known for assessing the operation of a WLAN. These probes act as wireless listening devices capturing all wireless packets in an area as they are transmitted. As the probes are generally only listening, they are not visible to other WLAN devices.
One use for such WLAN probes is for security, where network administrators employ them to detect and prevent unauthorised network access. Similarly, probes may also be used by hackers to detect accessible networks or networks with security vulnerabilities.
Another use of WLAN probes is to assess the operation of the WLAN. For example, Sniffer Wireless 4.6™ available from Sniffer Technologies Inc., which is a division of Network Associates of Santa Clara, Calif., an administrator can obtain certain basic information about the network performance, including for example how many packets were sent and at what speed, as well as how many data, management, control and encrypted frames have crossed the network in a given time. An administrator may also monitor individual details for individual stations on the WLAN including their transmission speed and signal strength.
The probes can also allow an administrator to analyse the individual packet sizes contained in the frames. Other products such as Airmagnet Reporter™ from Airmagnet Inc. of Sunnyvale, Calif., accept data from a probe and displays a performance value in the form of a percentage figure for WLAN utilization and assists an Administrator to identify WLAN congestion. It is believed, by the inventor of the present invention, that the percentage figure displayed is a mere calculation from the aggregation of the size of data packets received divided by the channel bandwidth (i.e. 11 Mbit/sec). Some probes allow the measurement of network efficiency/load by making attempts to transmit data on the network and measuring the time taken to do so.
A significant disadvantage of the existing probes is that they are generally wireless equivalents of wired packet network analysers, i.e. they analyse the packets transmitted rather than the frames in which they were transmitted. The limited performance values available do not provide sufficient and/or meaningful information about the quality of operation of the WLAN's to provide for sufficient information to truly assist a network manager or to facilitate admission control or quality of service provisioning schemes whereby usage of the network could be restricted as performance on the wireless medium decreases.